This study is of an immunosuppressive factor identifiable in hyperimmune sera, ascites of MOPC-315 tumors, and occasional normal mouse sera. An attempt will be made to isolate and characterize physicochemically and biologically the immunosuppressive factors from these various sources, using the Mischell-Dutton system of in vitro antibody production as the indicator system. For isolation of the factors, gel filtration, ion exchange chromatography including hydroxyl appetite, and preparative polyacrylamide electrophoresis will be used. For characterization of the biologic properties of the immunosuppressive factor, its behavior in excess of adherence cells, its effect on cell clustering, its effect on T-independent antigens, and the kinetics of its actions will be used, and the immunosuppressive factors from the three sources will be compared. It is believed that the immunosuppressive factor under investigation may be a substance of wide biologic importance, part of the normal mechanisms operative in immunoregulation of antibody responsiveness.